1. Field of the Invention
The present invention generally relates to a sensorless driving method for a brushless DC motor, in particular, to a sensorless driving method which achieves low noise by using the terminal voltage and central phase voltage of a motor.
2. Description of Related Art
A position sensor (for example, a Hall device) in a brushless DC motor is used for replacing the commutation structure (for example, a commutator and a carbon brush) in a conventional brush DC motor, therefore the brushless DC motor has such advantages as low noise, long life, and small volume etc. Besides, brushless DC motor has high rotating speed and requires no maintenance because no commutator or carbon brush is used; therefore brushless DC motor has replaced brush DC motor in automatic servo control systems. Moreover, brushless DC motor is a very good driving apparatus regarding its control performance and stability, thus, brushless DC motors are broadly applied to both industrial fields and household appliances. Furthermore, brushless DC motor plays a very important role in post PC era when it is applied to information appliances such as CD-ROM and DVD player etc.
In most existing driving methods for a brushless DC motor, a sensor (for example, a Hall device, an optical encoder, or a revolver) is usually used for detecting the position of a motor rotor, and a driver can only output suitable commutating signals for driving the brushless DC motor appropriately with such a position sensor installed. The performance of commutating control will directly affect the performance of close-loop position control and speed control. Therefore, to increase the resolution of the sensor and the accuracy of commutating control, both the volume and cost of the system with the sensor installed are increased.
The design of motor is going towards miniaturization, high precision, and low noise inevitably, and the space taken by the position sensor has become a major obstacle for reducing the volume of the motor. In addition, a position sensor is sensitive to temperature and noise and has life problem, which reduces the reliability of the sensor and the commutating control, and accordingly restricts the application of the motor relatively.
Accordingly, by using a sensorless driving technique, the time and effort for locating the accurate position of a sensor can be saved, and based on various restrictions of a sensor described above, sensorless driving technique is a very promising driving technique.
A digital phase shifter is disclosed in ROC Patent NO. 591882. According to this disclosure, the phase shifter includes two counters identically greater than 0, and these two counters are used for estimating the phase delay time that a motor rotates an electrical angle of 30°. The theory thereof will be described below. First, a first and a second counter are respectively defined as a positive/negative edge triggered counter, and the increment and decrement thereof are respectively ri and rd. When a positive edge of the induced electromotive force occurs (the voltage level thereof is changed from negative to positive), the first counter increases by ri until a negative edge of the induced electromotive force occurs (the voltage level thereof is changed from positive to negative), and here the value counted by the first counter is the time required for the motor to rotate an electrical angle of 60°. Accordingly, the time for the motor to rotate an electrical angle of 30° can be delayed by only changing the count-down rate of the first counter to two times of ri (i.e. rd=2ri) when a negative edge of the induced electromotive force occurs, and it is the actual commutating point when the first counter counts down to zero. The working theory thereof while a negative edge of the induced electromotive force occurs is similar except that it is accomplished by the second counter.
A digital phase shifter which achieves 90° phase delay is disclosed in U.S. Pat. No. 5,886,486. The phase shifter in this disclosure includes six counters Pa, Na, Pb, Nb, Pc, and Nc. First, signals Sa, Sb, and Sc are respectively defined as the output signal after comparing three phase terminal voltages with central phase voltage. Pa, Pb, and Pc respectively count the time while the signals Sa, Sb, and Sc being logic high, and Na, Nb, and Nc respectively count the time while the signals Sa, Sb, and Sc being logic low. Each counter counts the time for the motor to rotate an electrical angle of 180°. If the counter Na is used for counting the time while the signal Sa being high voltage level when the signal Sa is changed from high voltage level to low voltage level (a zero-crossover of the induced electromotive force occurs), when the counter Na counts to half of the value of counter Pa, which means the motor has rotated an electrical angle of 90°, a phase shifting signal is output, the counter Pa is reset to zero, and the counter Na continues to count. If the counter Pa is used for counting when the signal Sa changes from low voltage level to high voltage level, a commutating signal is output and the counter Na is reset to zero when the counter Pa counts to half of the value of counter Na. The counters Pb, Nb, Pc, and Nc perform similar operations, and accordingly, six commutating signals for a six-step square wave can be estimated by using the six counters.